Tapered birdcage resonator for improved homogeneity in MRI

ABSTRACT

A method for creating improved homogeneity in magnetic flux density in a radio frequency resonator for magnetic resonance imaging and spectroscopy of the human head. A tapered birdcage resonator is also provided. The tapered birdcage resonator includes two electrically conductive rings and a plurality of rods or conductor legs. The first electrically conductive ring forms an inferior end of the coil. The plurality of legs extends from the first electrically conductive ring. Each of the plurality of legs has a linear portion and a tapered portion. The second electrically conductive ring forms a superior end of the coil and is connected to the tapered portion of the plurality of legs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.60/109,831 filed Nov. 25 1998.

FIELD OF THE INVENTION

This invention relates to a radio frequency resonator coil suitable formagnetic resonance imaging. The radio frequency resonator coil is a wellknown device that is useful for imaging regions of a human patient, suchas the head and neck. Other useflI applications for this inventioninclude imaging other portions of the human anatomy or for imagingnon-human subjects.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging (MRI) is an imaging technique that may beused to produce high resolution images of the interior of the humanbody, for example, for the purpose of medical diagnosis. Interior imagesof the human body are produced based on the absorption and emission ofenergy in the radio frequency range of the electromagnetic spectrum. Theemission of energy is then correlated to the attenuation spectrum of thevarious tissues in the human body.

Typically, magnetic resonance imaging is performed by placing a patientin a constant magnetic field, B₀. A radio frequency excitation pulse maythen be transmitted to the examined region. The excitation pulses causemagnetic moment alignment of atomic nuclei. Upon removal of theexcitation pulses, the nuclear moments begin to realign with theconstant magnetic field, B₀. During this realignment period, the nuclearmoments emit radio frequency signals characteristic of the magneticfield and of the particular chemical environment in which the nucleiexist.

An RF coil may be used to both transmit the excitation pulses andreceive the signals from the nuclei. Alternatively, one RF coil may beused to transmit the excitation pulses and another separate coil toreceive the signals from the nuclei. RF coils of various types areknown.

For purposes of the following, coordinate references (X, Y and Z) aresometimes discussed. As used herein, the Z-axis is oriented along themain magnetic field, B₀. When reference is made to homogeneity, it is inreference to the homogeneity of the field pattern along the Z-axis, atany point in the directions of the Y and X axes.

One type of RF coil, the birdcage resonator coil, is known for itshomogeneity in the XY image plane, as well as its high signal-to-noiseratio performance. Such devices, however, may still exhibitshortcomings. For example, in the XZ and YZ image planes, the fieldpattern is not as homogeneous as it may be in the XY image plane.Homogeneity in the XZ and YZ image planes is desirable for highresolution imaging. Increased homogeneity in the YZ or XZ image planesof the conventional birdcage coils may be achieved by extending thelength of the coil. This increases the volume of the coil, however, andtherefore reduces the signal-to-noise performance of the coil.

U.S. Pat. No 5,602,479 to Srinivasan et al. (“the Srinivasan patent”),the contents of which are incorporated herein by reference, describes abirdcage coil where the z-axis conductors converge to a single point atthe superior end of the coil. Birdcage coils having thischaracteristic—the convergence of the z-axis conductors to a singlepoint—may be referred to herein as “dome resonators.” Srinivasan furthershows combinations of conductors using overlapped resonators, where theconductor junction points of all the z-axis conductors of the domeresonator are merged to a common point at one end which forms a virtualground point.

A disadvantage of the devices shown in the Srinivasan patent is that theconvergence of the junction points to form a virtual ground forces theresonator magnetic flux density to be highest at the convergent domeend, and fall off non-homogeneously as a function of distance from thedome end. The Srinivasan patent also discusses methods of distributingcurrents in a dome resonator between first and second rings or changingthe convergence angle to influence homogeneity. The disadvantages ofthese methods, however, are that they require careful componentselection to distribute the currents appropriately, they result in acoil that is too long for optimum signal-to-noise ratio, and theyrequire a virtual ground convergence point.

An article entitled “A 3×3 Mesh Two Dimensional Ladder Network Resonatorfor MRI of the Human Head,” Meyer et. al., Journal of MagneticResonance, Series B 107, 19-24 (1995), the contents of which areincorporated herein by reference, demonstrates that the resonant modesof the planar coil pairs are degenerate modes, and provides a saddlepair coil configuration at the non-domed end of the resonator, and acoplanar loop configuration at the domed end. Disadvantages of such anarrangement may include non-optimized homogeneity, and reducedsignal-to-noise performance compared to devices made in accordance withthe present invention.

An article entitled “A Novel Multi-segment Surface Coil forNeuro-Functional Magnetic Resonance Imaging,” Lin et. al., MagneticResonance in Medicine, 39: 164-168 (1998), the contents of which areincorporated herein by reference, demonstrates the resonant modes of theplanar coil pairs are degenerate modes, as two orthogonal half-loopmodes and causes the magnetic flux density to be most concentrated atthe closed superior end or apex end of the resonator, thereby producingstrong inhomogeneity. The device described by Lin et al. also requiresclosed end convergence at the most superior end of the conductors forthe resonance modes to operate.

It is desirable to have improved homogeneity and, simultaneously, highsignal-to-noise ratio performance, particularly in the XZ and YZ imageplanes, over that provided by known coils and methods. It is alsodesirable to improve homogeneity without impairing the signal-to-noiseperformance of a birdcage coil. It would therefore be desirable to havean improved birdcage coil.

SUMMARY OF THE INVENTION

This invention describes a novel apparatus and method for improving theXZ and YZ inhomogeneity of birdcage coils, without effecting the lengthof the coils.

The most homogeneous section of a conventional birdcage coil is withinthe center third of the length of the structure. Therefore to achievemaximum homogeneity within this region, a length to diameter ratio oftwo to one is typically used. The point of diminishing return forhomogeneity has empirically been shown to be when the length-to-diameterratio reaches two-to-one. To achieve maximum signal-to-noise ratioperformance for human head imaging however, the ratio of thelength-to-diameter is approximately one to one. Therefore, prior to thepresent invention, conventional birdcage coils could not simultaneouslyprovide maximum signal-to-noise performance for human head imaging whileachieving maximum homogeneity, especially over the region of the brain,because of the conflicting physical restraints placed on the coil bythese two requirements.

A preferred embodiment of the present invention meets the need for anMRI coil that provides the high signal-to-noise ratio performance foundin a normal length resonator coil combined with the homogeneity of along resonator coil. In particular, improved homogeneity is achievedwithout increasing the length of the coil, thereby maintaining a highsignal-to-noise ratio. Moreover, the preferred embodiments of thepresent invention meet the requirement of improved XZ and YZ image planehomogeneity for shorter, higher signal-to-noise ratio, birdcage coils bytapering the coil to optimize the magnetic field homogeneity. Thistechnique may provide an improved signal-to-noise ratio over even ashort birdcage coil of conventional design, especially in the region ofthe brain.

In accordance with a first aspect of the present invention, a method isprovided for increasing the homogeneity in the magnetic field withoutincreasing the length to diameter ratio of the coil. The method providesa coil with superior signal-to-noise performance and increasedhomogeneity in the XZ and YZ image planes.

In accordance with a second aspect of the present invention, a taperedbirdcage coil is provided. The coil structure includes criticallytapering the most superior end of the coil, and varying the radius ofthe taper and the end ring diameter, until the maximum homogeneity isrealized in the field pattern of the coil.

In accordance with a third aspect of the present invention, a taperedbirdcage coil is provided. The coil structure includes criticallytapering both the superior end of the coil and the inferior end of thecoil, and varying the radius of the taper and the end ring diameters,until the maximum homogeneity is realized in the field pattern of thecoil. The tapered birdcage coil provides improved homogeneity and goodsignal-to-noise ratio performance.

It is an object of the present invention to provide the homogeneity of alonger birdcage coil, with the signal-to-noise characteristic of ashorter birdcage coil.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention are illustrated byway of example, and not limitation, in the figures of the accompanyingdrawings, in which:

FIG. 1A depicts an electrical schematic of an embodiment of the presentinvention for an optimized transmit/receive tapered birdcage resonator;

FIG. 1B is an electrical schematic of a second embodiment of areceive-only tapered birdcage resonator;

FIG. 2A depicts a wire model for a prior art birdcage resonator, wherethe wire model may be used in a Biot-Savart software analysis program,in which the conductor geometry pattern represents the case of a domeresonator, where the conductors at the most superior end of the coilconverge to a common point;

FIG. 2B depicts a wire model of a tapered birdcage resonator used in theBiot-Savart software analysis for the conductor geometry pattern of anovel tapered birdcage resonator, where the most superior end of thecoil is dimensionally tapered to optimize the field pattern homogeneityin the XZ and YZ image planes, without sacrificing signal-to-noiseperformance;

FIG. 2C depicts a wire model for a prior art birdcage resonator, wherethe wire model may be used in the Biot-Savart software analysis for theconductor geometry pattern of a standard cylindrically shaped birdcageresonator;

FIG. 3 illustrates a cross sectional plot of the iso-intensity lines ofthe magnetic flux density (db) as a function of position with respect tothe conductor geometry pattern for the case of a dome resonator as shownin FIG. 2A;

FIG. 4 illustrates a cross sectional plot of the iso-intensity lines ofthe magnetic flux density (dB) as a function of position with respect tothe conductor geometry pattern of the tapered birdcage resonatorillustrated in FIG. 2B, where the most superior end of the coil isdimensionally tapered to optimize the field pattern homogeneity in theXZ and YZ image planes, without sacrificing signal-to-noise performance;

FIG. 5 illustrates a cross sectional plot of the iso-intensity lines ofthe magnetic flux density (dB) as a function of position with respect tothe conductor pattern geometry of the standard cylindrically shapedbirdcage resonator illustrated in FIG. 2C;

FIG. 6 depicts a wire model of a conductor pattern geometry for atapered birdcage resonator in which both the superior and inferior endsof the coil are critically tapered;

FIGS. 7A through 7C show the structural characteristics, including aradius of the arc used for the legs, of a preferred embodiment of atapered birdcage resonator;

FIGS. 8A and 8B illustrate an embodiment of a high resolution taperedbrain coil where the length of the coil is less than the large diameterof the large end ring and the legs are radially tapered; and

FIG. 9 is an electrical schematic for the embodiment of the taperedresonator shown in FIGS. 8A and 8B.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT(S) OF THEINVENTION

The presently preferred embodiments of the present invention will now bedescribed with reference to the FIG.s, in which like elements arereferred to by like numerals. FIG. 2A depicts a wire model for a priorart birdcage resonator in which the conductor geometry patternrepresents the case of a dome resonator. Dome resonators arecharacterized by the convergence of the z-axis conductors to a singlepoint. As shown in FIG. 2A, the z-axis conductors converge at the mostsuperior end of the coil to a common point 5. A disadvantage ofresonators characterized by this wire model is that the convergence ofthe junction point forms a virtual ground, thus forcing the resonatormagnetic flux density to be highest at the convergent dome end, and tofall off non-homogeneously as a function of distance from the dome end.

The magnetic flux density at points within the dome resonator wire modelshown in FIG. 2A may be studied using a Biot-Savart software analysisprogram, as is known to those skilled in the art. If the birdcage coilis driven in quadrature, then the magnetic flux density plots in the XZand YZ planes should be identical because the field pattern of the coilhas radial symmetry about the Z-axis. FIG. 3 illustrates a crosssectional plot of iso-intensity lines of the magnetic flux density (dB)as a function of position with respect to the conductor geometry patternfor the case of a dome resonator as shown in FIG. 2A. The plot shown inFIG. 3 may be generated by applying Biot-Savart's Law to the domeresonator wire model shown in FIG. 2A.

As illustrated in FIG. 3, the iso-intensity lines are well spaced overthe middle third of the length of the dome resonator, but become closelyspaced at both ends; the dome end creates a field that is too high inrelative intensity, and the inferior end creates a field that drops offin relative intensity. If the domed resonator were to be used forstudies of the head, for example, it would be desirable to improve thehomogeneity, especially in the region toward the superior end of theresonator.

FIG. 2C depicts a wire model for a prior art birdcage resonator, theconventional cylindrical birdcage resonator. The wire model may be usedin a Biot-Savart software analysis to study the magnetic flux densitywithin the coil. FIG. 5 illustrates a cross sectional plot of theiso-intensity lines of the magnetic flux density (dB) as a finction ofposition with respect to the conductor pattern geometry of the standardcylindrically shaped birdcage resonator illustrated in FIG. 2C. The plotshown in FIG. 5 may be generated by applying Biot-Savart's Law to thecylindrical resonator wire model shown in FIG. 2C. Like the plot in FIG.3, the plot shown in FIG. 5 demonstrates that the iso-intensity linesare well spaced over the middle third of the length of the resonator,but become closely spaced at both ends.

In accordance with preferred embodiments of the present invention, abirdcage coil having improved homogeneity and a method for designingsuch a coil are provided. In particular, improved homogeneity incomparison with the types of coils shown in FIGS. 2A and 2C is achieved,without sacrificing signal-to-noise ratio. The preferred embodiments areespecially useful for higher resolution and/or faster image acquisitiontime in studies of the human head, for example.

In accordance with a preferred method for designing a birdcage coil, theplacement and geometry of the coil conductors is established byiteratively applying a Biot-Savart model to a corresponding wire modelof the coil. The Biot-Savart model is used to calculate the magneticflux density at all spatial locations based on the RF current throughthe coil conductors and the geometry of the coil structure. The magneticflux density, B, may be numerically determined based on Biot-Savart'sLaw, as follows:$\overset{\rightarrow}{B} = {\frac{\mu_{o}}{4\pi}I{\int_{- \infty}^{\infty}\quad \frac{{\overset{\rightarrow}{l}} \times \left( {\overset{\rightarrow}{r} - \overset{\rightarrow}{a}} \right)}{{{\overset{\rightarrow}{r} - \overset{\rightarrow}{a}}}^{3}}}}$

where:

{right arrow over (B)} is the magnetic flux density (T);

{right arrow over (dl)} is an element of conductor in the direction ofthe current;

{right arrow over (a)} is the position of the conductor; and

{right arrow over (r)} is the spatial position that the magnetic fluxdensity is being computed at.

The Biot-Savart model, which may be implemented in software such asMathCad or Matlab and is well known to those skilled in the art, may beutilized to compute the numeric values of the magnetic flux densityvalues for each spatial position. The values of the magnetic fluxdensity are preferably graphically displayed as iso-intensity lines ofthe magnetic flux density, B, as a function of position with respect tothe coil conductors. In such a plot, a numerical value associated withthe iso-intensity lines depicts the relative strength of the magneticflux density in space, and the lines of iso-intensity depict thehomogeneous value of magnetic flux density. This technique provides avisual representation of the expected imaging field pattern for a givencoil geometry.

In accordance with a preferred embodiment of the design method, theBiot-Savart model provides a useful tool for modeling the effects ofvarying the coil conductor geometry to alter the signal-to-noiseperformance and homogeneity, as further described below. This was thetool used to develop the embodiments of the tapered birdcage coilgeometry described herein. Other tools that are capable of determiningthe magnetic flux density values may alternatively be used.

For a birdcage resonator with N leg conductors, N/2 resonance modes areexhibited, (N/2-1 are degenerate, and one is non-degenerate. For anarbitrary n^(th) one of the legs, the current distribution for the twoquadrature modes follows a cosine current distribution as:$I_{n} = {I_{o}e^{i{(\frac{2\pi \quad n}{N})}}}$

where I_(o) is the current in the end ring. The direction of the currentdepends on its azimuthal direction. The principle mode is selected andhas two linear modes orthogonal to one another and provides ahomogeneous field at the coil center. These two linear, orthogonal modesare matched to the proper impedance (50Ω), and either quadraturecombined at the output and fed to the system preamplifiers or fed toseparate system preamplifiers and receiver channels.

Because both the domed birdcage coil and the cylindrical birdcage coilexhibit less than optimal homogeneity at points located away from thecenter of the coil, as shown in FIGS. 3 and 5 one would expect that anyintermediate design would have similar deficiencies. However, thepreferred embodiments provide a tapered birdcage coil having improvedhomogeneity, particularly and advantageously, toward the superior end ofthe coil.

A preferred embodiment of a method for designing a coil exhibitingimproved homogeneity will now be described. The method makesadvantageous use of the Biot-Savart model described above. The methodbegins with the conventional cylindrical birdcage wire model shown inFIG. 2C. As described above and as shown in FIG. 5, the conventionalcylindrical birdcage exhibits good homogeneity over the middle third ofthe coil, but the magnetic flux density rolls off as one approaches theends from the center of the coil. By definition, the cylindricalbirdcage resonator has two end rings of equal diameter.

In accordance with the preferred embodiment of the design method, a coilwith improved homogeneity is designed by iteratively adjusting: 1) therelative sizes of the end rings, aiid 2) the radius of the taper formedby connecting the leg conductors to the end rings. For each iteration ofthe leg conductor and end ring geometry, the Biot-Savart model may beused to acquire information about the homogeneity of the magnetic fluxdensity. The conductor and end ring geometry are then adjusted basedupon the results of applying the Biot-Savart model until the desiredhomogeneity is obtained. In this example, it is assumed that the lengthof the leg conductors is not a variable so that the signal-to-noiseperformance of the coil is maintained. Nonetheless, for applications inwhich the signal-to-noise ratio is less important, the length of the legconductors may be used as an additional variable in the design process.The wire model shown in FIG. 2B, which has the iso-intensity plot shownin FIG. 4, is a product of this method.

As an alternative to the Biot-Savart modeling tool, experimentalverification of the field pattern for a particular coil geometry may beused as an input for the optimization method set forth above. As afurther alternative, a combination of experimental verification andBiot-Savart modeling may be used. For example, early iterations mayutilize the BiotSavart model to acquire information about thehomogeneity of the magnetic flux density, with later iterations in theprocess relying upon experimental verification.

FIGS. 1A and 1B are electrical schematics for two embodiments of atapered birdcage resonator in accordance with the present invention. Thetapered birdcage resonator embodiment shown in FIG. 1B is a “receiveonly” resonator, i.e. the tapered birdcage resonator does not apply theexcitation pulses, but rather the coil is used with an external transmitcoil. The coil shown in FIG. 1B includes decoupling networks to activelydecouple the coil during transmit cycles, which technique is well knownto those skilled in the art. As described further below, however, thetapered birdcage resonator may alternatively be a transmit/receive coil,as shown in FIG. 1A.

The tapered birdcage resonator can take form in various configurationsof components and component placement. The drawings, which illustrate aband pass configuration, are for purposes of illustrating preferredembodiments and are not to be construed as limiting the invention. Inparticular, the components alternatively may be selected and placed, ina manner known to those skilled in the art, to create a low pass or highpass configuration of the tapered birdcage resonator.

Referring now to FIGS. 1A and 1B for the band pass configuration, thecapacitors on the small end ring 20 CS 22(a)-22(d) are selected toachieve proper impedance match using a balanced drive technique over 180degrees of the end ring 20 for each of the two quadrature modes. Thedrive points are at virtual ground by splitting the end ring capacitorsCS 22(a)-22(d) into two equal values CS′ 24(a)-24(h) that are double thevalue of a single end ring capacitor CS 22(a)-22(d). In a preferredembodiment as shown in FIG. 1B, the value of CS 22(a)-22(d) is 110 pFand therefore the value of CS′ 24(a)-24(h) is 220 pF. Capacitance isdistributed in the legs 30, CT 32(a)-32(h) and CL 34(a)-34(h), tominimize any electric field patient coupling to the coil.

For the embodiment shown in FIG. 1B, a total desired adjustable range ofcapacitance in the legs 30 is 34 pF to 51 pF. Therefore, if CL34(a)-34(h) is 33 pF then the range of the trim capacitor CT 32(a)′32(h)in parallel with CL 34(a)-34(h) would be 1 pF to 16 pF, as shown in FIG.1B. Tuning of the coil is achieved by varying the capacitance in thelegs CT32(a)-32(h) of the tapered resonator equally. The capacitors ofthe large end ring CLE 42(a)-42(h) are selected to minimize the electricfield patient coupling to the coil. In a preferred embodiment, the valueof CLE 42(a)-42(h) is 89 pF. Because the embodiment shown in FIG. 1B isa receive-only coil, the diodes D1 and D2 and inductors provide transmitdecoupling.

The drive capacitors CD 26(a) & 26(b) in FIG. 1A compensate for anyinductive length in the balanced drive. Capacitance CD in FIG. 1A isC1/C2/C3 in series with C10 for one mode, and C4/C6/C8 in series with C5for the other mode. The crossover PCB simply provides the connections tothe coil from the feedpoints, A/B and C/D.

Referring again to FIG. 2B, the wire model used in the Biot-Savartanalysis for the conductor geometry pattern of a tapered birdcageresonator is shown. The most superior end of the coil 10 isdimensionally tapered to optimize the field pattern homogeneity in theXZ and YZ image planes, without sacrificing signal-to-noise performance.The magnetic flux density within the dome resonator, tapered birdcage,and the cylindrically shaped birdcage resonator shown in FIGS. 2Athrough 2C were calculated using Biot-Savart's Law. FIGS. 3, 4, and 5contain the results of the Biot-Savart analysis for the wire models ofFIG. 2A, 2B and 2C, respectively.

FIG. 4 illustrates a cross sectional plot of the iso-intensity lines ofthe magnetic flux density (dB) as a function of position with respect tothe conductor geometry pattern of the tapered birdcage resonator shownin FIG. 2B. As shown in FIGS. 2B and 4, the most superior end of thecoil is dimensionally tapered to optimize the field pattern homogeneityin the XZ and YZ image planes, without sacrificing signal-to-noiseperformance. Notably, the iso-intensity plot for the tapered birdcageresonator illustrates improved homogeneity as reflected by the increasedspacing between iso-intensity lines, in the region toward the superiorend of the resonator, and the large highly uniform region within asingle iso-intense band.

FIGS. 1A and 1B show electrical schematics for implementing the wiremodel of FIG. 2B. FIGS. 7A through 7C show the structuralcharacteristics, including a radius of an arc used for the legs, of apreferred embodiment of a tapered birdcage resonator.

FIGS. 7A and 7C show a tapered birdcage resonator having a large endring 40, a small end ring 20, and a plurality of conductor legs 30. Forthis embodiment, the large end ring 40 has a diameter of approximately11.25 inches, the small end ring 20 has a diameter of approximately 5.67inches, and the leg conductors 30 extend approximately 11.59 inches fromthe large end ring 40 to the small end ring 20. Thus the ratio of thelength of the resonator to the diameter is 11.59 to 11.25 approximately1 to 1.

A detail of a typical leg conductor 30 is also shown in FIG. 7B. The legconductor 30 includes a linear portion 35 and an arced portion 36, whichmay also be referred to herein as the tapered portion of the legconductor 30. For the embodiment shown in FIGS. 7A through 7C, thelinear portion 35 is 7.24 inches in length and the radius of the taperedportion is 5.5 inches.

The tapered birdcage resonator shown in FIGS. 7A through 7C may be usedfor neurovascular or head imaging. The dimensions of this embodimentmake it particularly useful for imaging areas of interest within thehuman head. Other dimensions of the tapered birdcage resonator mayalternatively be used for other applications or other regions ofinterest. This embodiment of the tapered birdcage resonatoradvantageously exhibits improved homogeneity, particularly toward thesuperior (small end ring 20) end of the resonator.

An alternate embodiment of the invention is illustrated in FIG. 6, whichincorporates critically tapering both the most superior and inferiorends of the coil. For this embodiment, the radius of the arc of thetaper and the end ring diameter are preferably varied until maximumhomogeneity is realized in the field pattern of the coil. This also willresult in optimizing the homogeneity throughout the entire length of thecoil.

A second alternate embodiment of the invention includes using othergeometric conductor patterns to form the arc at the tapered end, such asangled linear segmented sections, eccentrically shaped arc, or any othergeometric realization which forms an arc at the tapered end. The desiredlevel of homogeneity may be achieved by calculating the magnetic fluxdensity using a Biot-Savart model. Adjustment to the wire model may bemade and a new magnetic flux density calculation may be made. Using thisiterative procedure, the desired homogeneity resulting from a variety ofgeometric shapes for the wire model may be developed.

A third alternate embodiment of the invention includes using end-ringswhich are tapered larger rather than smaller (relative to the diameterat the center of the resonator) to provide a concentrated magnetic fluxdensity within the region centered within the resonator. The magneticflux density constriction would be similar to the result realized insolenoid shaped magnet plasma pinch columns.

A fourth alternate embodiment of the invention includes criticallyoverlapping other resonator structures at the non-tapered, or mostinferior, end of the tapered birdcage resonator in a multi-coil phasedarray arrangement, whereby the tapered birdcage and other resonatorsform the coil elements of the array. These include birdcage resonators,ladder resonators, loops, Helmholtz pairs, saddle pairs, etc. Forexample, a four-element array may be constructed by combining twocervical spine coils and an anterior neck coil with the tapered birdcageresonator. Cervical spine coils are described in U.S. Pat. No.5,196,796, the contents of which are incorporated herein by reference.The anterior neck coil may be an Anterior Neck Coil, as available fromMedrad, Inc. of Indianola, Pa., or any equivalent thereto. Thisarrangement is particularly useful for neurovascular imaging. As afurther example, and in accordance with a preferred embodiment, afour-element array may be constructed by combining two cervical spinecoils and an anterior neck array coil with the tapered birdcageresonator. For this embodiment, the RF output of the two cervical spinecoils may be combined at the RF level and applied to a single receiver,the tapered birdcage may be applied to a receiver, and the anterior neckarray may include a superior neck coil and an inferior neck coil, eachof which may be applied to a receiver.

A fifth alternate embodiment of the invention includes using a differentgeometry of the conductor legs to form an elliptical or other shapedresonator structure with respect to the XY imaging plane. The iterativedesign method described above may be used to determine a criticaltapering of one or both the ends of the coil, a radius of the arc of thetaper, and an end ring diameter that results in increased homogeneity inthe field pattern of the coil.

FIGS. 8A and 8B shows a tapered birdcage resonator in accordance withthe fifth embodiment. FIG. 8A, which is a view along the z-axis of thecoil, shows the conductor geometry pattern in the XY imaging plane. Inthe XY imaging plane, the resonator has an elliptical shape, with themajor diameter of the large end ring being 10.07 inches and the minordiameter of the large end ring being 9.27 inches. The small end ring hasa major diameter of 5.875 inches and a minor diameter of 5.086 inches.As shown in FIG. 8B, the radius of the taper towards the small end ringis 4.635 inches, and the radius begins 3.875 inches from the large endring.

Like the tapered birdcage resonator shown in FIGS. 7A through 7C, thetapered birdcage resonator shown in FIGS. 8A and 8B preferably has eightleg conductors. More or fewer leg conductors may alternatively be used.

In comparison with the embodiment shown in FIGS. 7A through 7C, itshould be observed that the embodiment shown in FIGS. 8A and 8B will,generally speaking, locate the leg conductors closer to the region ofinterest, such as the human head. It should also be noted that thelength to diameter ratio for the embodiment shown in FIGS. 8A and 8B isactually less than 1 to 1. The reduced coil volume provides improvedsignal-to-noise performance in comparison to known resonators such asthe cylindrical birdcage and the domed birdcage. Although the embodimentshown in FIGS. 8A and 8B is preferably used to obtain high resolutionimages of the human brain, it may be used for other applications aswell.

FIG. 9 is an electrical schematic for the embodiment of the taperedresonator shown in FIGS. 8A and 8B, in which the tapered resonator is aband pass, high-resolution brain coil driven in quadrature mode. The rodand end nng capacitors are selected to ensure that the resonantfrequencies of the two modes of the coil remain identical, and theelectrical isolation betwixt the two quadrature modes of the coilremains high despite the distortions to the physical structure symmetrycaused by the non-circular cross section. The capacitors CT 132 (b),(d)-(h) and CL 134 (b), (d)-(h), on legs 130 (b), (d)-(h) are selectedto achieve a proper impedance match using a balanced drive techniqueover 180 degrees for each of the two quadrature modes. The in-phase (I)drive point is at leg 130(a) through capacitors 136(a), CD 137(a), and138(a). The out-of-phase (Q) drive point is at leg 130(c) throughcapacitors 136(c), and 138(c). Capacitance is distributed in the legs130 (b), (d)-(h), through CT 132 (b), (d)-(h) and CL 134 (b), (d)-(h) tominimize any electric field patient coupling to the coil. Althoughcapacitors CT 132 (b), (d)-(h) and CL 134 (b), (d)-(h) are shown inparallel in FIG. 9, they may alternatively be in series provided theappropriate values for the capacitors are chosen. Tuning of the coil isachieved by varying the capacitance in the legs CT 132 (b), (d)-(h) ofthe tapered resonator.

In a preferred embodiment as shown in FIG. 9, the value of CS122(a)-122(h) on the small end ring is 820 pF. Capacitors 136(a), (c),and 138(a), (c) are 47 pF. The drive capacitor CD 137(a), 8.2 pF, isused to compensate for any inductive length in the balanced drive. Thecapacitors ofthe large end ring CLE 142(a)-142(h) are selected tominimize the electric field patient coupling to the coil. In a preferredembodiment, the value of CLE 142(a)-142(h) is 190 pF.

A sixth alternate embodiment of the invention includes an electricalimplementation of the tapered birdcage resonator using variousconfigurations of components and component placement. This will create alow pass, band pass or high pass configuration of the tapered birdcageresonator. For example, the placement of the components may be changed,or even the use of other reactive components such as inductors may beused to practice the invention.

A seventh alternate embodiment of the invention includes electricallydriving the tapered birdcage resonator either linearly, in quadrature,or in phased array mode at either of the tapered or non-tapered endrings, or electrically driving the resonator on any of the legs of theresonator. For example, the tapered birdcage coil may be a quadrature,transmit/receive, eight rod, quasi-bandpass tapered birdcage resonator.The advantage of this configuration is that the coil has a smallerdiameter and reduced length to bring the enclosed volume to a practicalminimum. This configuration may be made using a quadrature hybrid thatinterfaces this coil element as a conventional transmit/receive deviceoperating in a quadrature similar to the GEMS Signa Quadrature HeadCoil.

To operate the coil as a linear coil, the coil will have a single RFoutput for connection to the MRI system. The current distribution in therods, or conductor legs of the coil, is sinusoidal, with the currentopposite the drive point in the reverse direction (equal magnitude) andthe current in the rods at +90° and −90° equal to zero. If on the otherhand, the coil is to operate in the quadrature mode, a second drivepoint, rotated by 90° from the first, is added (see FIG. 1A). The seconddrive point is at the zero current point for the first linear mode ofthe coil. The first drive point is at the zero current point for thesecond drive linear mode of the coil. Linear and quadrature operation ofMRI coils are topics that are well known to those skilled in the art.Either capacitive or inductive coupling may be used.

Another possible configuration is to use the tapered birdcage in thephased array mode as a transmit/receive coil. A proper phased arraycable is required to apply power to two outputs only in quadrature, andto receive on two receivers. The transmit mode operates the coil elementas a conventional quadrature birdcage. The two quadrature outputs drivetwo Phased Array ports.

The coil may have two distinct operating modes: one for highest imagequality, and one for rapid [single receiver] acquisitions. For thehighest image quality, the basic coil will operate as a two elementphased array receiver coil where the two channels are derived from thetwo quadrature outputs from the tapered birdcage element. The transmitmode operation will be accomplished by applying power to the sametapered birdcage element. When single receiver operation is desired forrapid acquisitions, the two coil quadrature modes will be combined priorto being input to the MRI system, and directed to a receiver input.Selection of the two modes may be made by the designation of the coilname, and the resulting Port Enable Mask designated by a configurationfile run on the MRI system.

In order to increase the uniformity and superior-to-inferior coverage ofthe transmit field from the tapered birdcage element, a spatiallyselective loading device may be used to attenuate only the transmitfield in a spatially appropriate manner. This will reduce the RFmagnetic field intensity B1 in a pattern that will endeavor tocompensate for the increased efficiency of the coil element in thecentral and superior regions of the coil. The field absorption devicewill be active only during the transmit mode operation of the coil.Therefore, it will not have any negative effect upon the receive mode RFmagnetic field sensitivity of the coil.

Alternatively, the coil can be used in a single receiver mode only.However, this prevents the coil from potentially benefiting from thephased array technology. The advantages of phased array include theability to equally load the two quadrature modes of the coil via thepreamplifier decoupling concept, as described by P. Roemer, “The NMRPhased Array,” Journal of Magnetic Resonance (Nov. 1990), and to utilizethe phase insensitive combining technique employed by the GE Signasystem for phased array acquisitions. As described above, the taperedbirdcage coil may also be set up as a receive-only coil. However, thereceive-only coil may encourage the generation of Annefact typeartifacts, and potentially extend scan times due to the need for the NoPhase Wrap option. Additionally, the use of decoupling networks on thecoil for receive-only operation will lower the available signal-to-noiseratio performance due to the inherent losses in the needed RF switchingdiodes.

Unlike prior coil designs that resulted in a coil that is too long foroptimum signal-to-noise ratio or that resulted in a virtual groundconvergent point causing field inhomogeneity, the preferred embodimentsprovide improved homogeneity in the XZ and YZ image planes whilemaintaining an optimum signal-to-noise ratio.

In view of the wide variety of embodiments to which the principles ofthe present invention can be applied, it should be understood that theillustrated embodiments are exemplary only, and should not be taken aslimiting the scope of the present invention. For example, the steps ofthe design method may be taken in sequences other than those described,and more or fewer elements may be used than are described. In addition,although reference is made herein to the GEMS Signa MRI system, othersystems having similar capabilities may alternatively be used to receiveand process signals from the coils described above.

We claim:
 1. A coil for magnetic resonance imaging comprising: a firstelectrically conductive ring forming an inferior end of the coil foraccess by a test subject, the first electrically conductive ring havinga first diameter; a plurality of legs extending from the firstelectrically conductive ring, each of the plurality of legs having alinear portion and a tapered portion; and a second electricallyconductive ring forming a superior end of the coil and having a seconddiameter which is different than the first diameter, the secondelectrically conductive ring being connected to the tapered portion ofthe plurality of legs, wherein the tapered portion of the plurality oflegs provide the coil with a substantially homogenous field pattern. 2.The coil of claim 1 further comprising a plurality of reactiveelectrical components connected within the electrically conductive ringsand the legs.
 3. The coil of claim 1, wherein the second electricallyconductive ring has a diameter that is smaller than a diameter of thefirst electrically conductive ring.
 4. The coil of claim 3, wherein thetapered portion of the plurality of legs has a radius that is selectedto maximize homogeneity in a field pattern of the coil.
 5. The coil ofclaim 4, wherein the field pattern is a magnetic flux density in atleast one of an XZ and a YZ imaging plane.
 6. The coil of claim 1,wherein the tapered portion of the plurality of legs comprises at leastone angled linear segmented section.
 7. The coil of claim 1, wherein thefirst electrically conductive ring and the second electricallyconductive ring are circular.
 8. The coil of claim 1, wherein the firstelectrically conductive ring and the second electrically conductive ringare elliptical.
 9. The coil of claim 1, wherein at least one of theelectrically conductive rings is tapered larger relative to the centerof said coil to provide a concentrated magnetic flux density within aregion centered within the coil.
 10. The coil of claim 1, furthercomprising at least one additional magnetic resonance (RF) coilpositioned to at least partially overlap the coil.
 11. The coil of claim1, wherein the coil is a receive only coil.
 12. A method as claimed inclaim 11, wherein the improved homogeneity of the magnetic flux densityis determined by applying a Biot-Savart model to the wire model.
 13. Amethod as claimed in claim 11, wherein the homogeneity of the magneticflux density is determined by experimental verification.
 14. The coil ofclaim 1, wherein the coil is a transmit/receive coil.
 15. The coil ofclaim 1, wherein a ratio of a length of the legs to a diameter of thefirst electrically conductive ring is approximately 1:1.
 16. The coil ofclaim 1, wherein the linear portion of the plurality of legs is attachedto the first electrically conductive ring, and the second diameter issmaller than the first diameter.
 17. The coil of claim 1, wherein thecoil is a quadrature coil.
 18. The coil of claim 17, wherein thequadrature coil is a phased array quadrature coil.
 19. A coil formagnetic resonance imaging comprising: a first electrically conductivering forming an inferior end of the coil; a second electricallyconductive ring forming a superior end of the coil; and a plurality oflegs extending between the first electrically conductive ring and thesecond electrically conductive ring, each of the plurality of legshaving a predetermined length, a linear center portion and a taperedportion at each end, wherein a ratio of the length to a diameter of thecoil is approximately 1:1.
 20. A method of making a birdcage resonatorhaving a plurality of legs to provide improved homogeneity whilemaintaining signal-to-noise performance, the method comprising the stepsof: constructing a wire model of the birdcage resonator; calculating amagnetic flux density within the birdcage resonator; and adjusting atleast one of an end ring diameter and a radius of taper the plurality oflegs to improve homogeneity of the magnetic flux density.
 21. A methodas claimed in claim 20, further comprising the step of adjusting avolume measure of the resonator to improve a signal-to-noise ratioassociated with the resonator.